1. Field of the Invention
The present invention relates to a developing roller, and a method for producing a developing roller used in contact with a photosensitive member set in an electrophotographic apparatus such as a receiving apparatus of a copying machine, a printer and a facsimile. Further, the present invention relates to a process cartridge and electrophotographic apparatus which use this developing roller.
2. Description of the Related Art
Japanese Patent Application Laid-Open Nos. H9-12192 and H8-208087 disclose a developing roller used in an electrophotographic apparatus which has a mandrel and an elastic layer including polyurethane obtained by reacting a polyol and a polyisocyanate on the periphery of the mandrel.
Developing rollers are required to have as a basic function a function of frictionally charging toner to a suitable level. However, in a low-temperature low-humidity environment, for example, in an environment with a temperature of 10° C. and a humidity of 10% RH, the frictional chargeability of the toner becomes too high in some cases. This can cause toner to locally adhere electrostatically to the developing roller surface. Consequently, spotty unevenness (hereinafter referred to also as “blotching”) may occur in the electrophotographic image.
When being set in the process cartridge or electrophotographic apparatus, developing rollers may be left standing in contact with a development blade or a photosensitive member in a static state for a long time period. In such a case, deformation which is not easily restored (hereinafter referred to as “permanent compression set”) may occur at the portions coming in contact with the development blade or photosensitive member of the developing roller having the above-described urethane resin layer as a surface layer. The permanent compression set especially tends to occur in a high-temperature high-humidity environment, for example an environment with a temperature of 40° C. and a humidity of 95% RH. This is thought to be for the reason the surface of the developing roller tends to become flexible in a high-temperature high-humidity environment.